Unfortunately, babies don't come with manuals. After my first baby was born I was told to feed her "on demand" and count her diapers. It all sounded pretty straightforward, until I was up all night nursing a crying baby. Later, I realized just how hard it can be to see a problem that is literally right in front of your face. I also learned that after their first few weeks, there are actually different red flags your baby isn't eating enough that you absolutely shouldn't ignore.

No matter how you feed your baby, it's natural to want to make sure they are getting enough to grow and thrive. Fortunately, there are some red flags that can help you identify a problem early and avoid long-term complications. According to the American Academy of Pediatrics (AAP), while it's normal for babies to lose up to 7 percent of their birth weight in their first couple of days of life, they usually regain their birth weight within their first week. After that they should gain 4 to 7 ounces per week. If not, it can be a sign of a problem.

Unfortunately, other red flags that your baby isn't eating enough are more subtle and might be a little harder to measure, especially for breastfeeding moms. According to Mayo Clinic, if your baby seems fussy after eating or seems like they are eating around the clock, they might still be hungry or not getting enough to eat. Also, your baby shouldn't be lethargic or difficult to wake, and should be alert and active at various times throughout the day. Fortunately, according to Jody Segrave-Daly, RN, MS, an International Board Certified Lactation Consultant (IBCLC) and NICU nurse, you can always take your baby in for a weight check if you think something is wrong or to make sure they are eating well.

The bottom line: if you have concerns about your baby's weight gain or how much they are eating, you should consult your doctor as soon as you can. Chances are, you can supplement and/or take steps to increase your supply, and if getting enough to eat is not causing their red flags, you can figure out if something else is going on.

They've Lost Or Aren't Gaining Weight

According to the AAP, watching your baby's weight is a great way to determine if they are getting enough to eat. Babies usually double their birth weight by 4 - 6 months of age, and triple it by their first birthday. On average, infants should gain 4 to 7 oz per week in their first 4 to 6 months, and 3 to 5 oz per week from ages 6 to 18 months.

But, if you don't have an infant scale at home, how do you know if your baby is gaining weight between appointments? Segrave-Daly recommends taking your baby in for weight checks regularly, even if you don't have a well-child appointment. "I am a huge fan of frequent weight checks the first six months and then monthly after that for the entire first year of life for exclusively breastfed babies. Moms weigh their babies at their doctor's office or at some hospital-sponsored mommy groups. If a scale is affordable, I suggest buying one before having your baby, as a scale is the best tool we have for adequate weight growth," Segrave-Daly told Romper via email.

She adds, "The problem is babies may gain enough during the first two weeks of life and then slowly being to lose weight, and it isn't caught until their two month weight check/immunization appointment. I also see babies who fall off the weight chart between 2 and 4 months of age frequently. A scale is everything."

They Are Fussy After Feeding

According to the Mayo Clinic, your baby should seem satisfied after eating. If they are still hungry after each feeding, or want to eat all of the time, you should take them in for a weight check or consult an medical professional about ways to assess or increase your supply.

While many people think that your baby "cluster-feeding" or wanting to eat all of the time is normal, most babies actually don't eat all day long. As the Fed is Best Foundation posted on their Facebook page, "cluster feeding, non-stop is never normal," and can be a sign that your baby is not getting enough to eat.

Segrave-Daly adds, "It important to know if a mother has any risk factors for the delayed onset of a full milk supply to know if timely supplementation is needed. I have educated families aboutsafe breastfeeding for 30 years now, and it works."

They Don't Have Many Wet Or Poopy Diapers

According to the AAP, your older baby should have six wet diapers and four poop-filled diapers each day. If they don't, you may want to take them in to a health care provider, just to make sure they are getting enough to eat. Other diaper-related red flags include dark yellow pee or red specks, which may be signs of dehydration.

They've Fallen Off Their Own Growth Curve

While it's easy to compare your baby to others, or to their older siblings, it's way more important to monitor their own progress on their own growth curve. Romper spoke with mom of two, Courtney Ferguson, about her experience having a baby diagnosed as "failure to thrive." She told Romper via email, "My son dropped off his curve at 4 months. After doing weighed feeds and finding he was raking in 36 ounces of breast milk in 24 hours, we realized my milk wasn’t calorie dense enough for him. We started giving him a few bottles of fortified milk each day, and he thrived. Even if your supply is great, and your baby is having appropriate output, if they’re not gaining weight like they should you need to look deeper."

They Are Sleepy Or Lethargic

While having a sleepy baby seems normal (and even great), according to the Mayo Clinic your baby should be "alert and active" in between feedings, and generally seem healthy. If they don't, it might be a sign that they aren't eating enough. According to the same site, it's important that parents learn warning signs and call their doctor if they think their baby might not be getting enough to eat.

They Aren't Meeting Developmental Milestones

According to emergency physician and brain Christie del Castillo-Hegyi, MD, Board Certified Emergency Physician and newborn brain injury researcher, not getting enough to eat can even impact brain development. She told Romper via email, "Weight gain is important for a baby’s brain development, as a child who is not gaining weight does not have the extra calories and nutrients necessary for their brain to grow."

According to the Children's Hospital of Philadelphia, your baby should meet developmental milestones, like rolling, smiling, and making sounds on time. If they aren't, it could be a sign they aren't getting enough breast milk or formula, or later on, that they aren't getting enough or the right types of solid food.

If your baby doesn't seem to be on track, you should definitely call their doctor. In addition to not getting enough to eat, other health conditions like intolerances and allergies, reflux, cystic fibrosis, celiac disease, or a heart defect might be to blame. So again, it's important to take your baby to the doctor, if you think there's a problem.

Check out Romper's new video series, Bearing The Motherload, where disagreeing parents from different sides of an issue sit down with a mediator and talk about how to support (and not judge) each other’s parenting perspectives. New episodes air Mondays on Facebook.